Download Development of the Theory of Plate Tectonics

Development of the Theory of Plate Tectonics
Close examination of a globe often results in the observation that most of the continents seem to fit together like
a puzzle: the west African coastline seems to snuggle nicely into the east coast of South America and the
Caribbean sea; and a similar fit appears across the Pacific. The fit is even more striking when the submerged
continental shelves are compared rather than the coastlines. In 1912 Alfred Wegener
(1880-1930) noticed the same thing and proposed that the continents were once
compressed into a single protocontinent which he called Pangaea (meaning "all lands"),
and over time they have drifted apart into their current distribution. He believed that
Pangaea was intact until the late Carboniferous period, about 300 million years ago,
when it began to break up and drift apart. However, Wegener's hypothesis lacked a
geological mechanism to explain how the continents could drift across the earths surface
as he proposed.
Searching for evidence to further develop his theory of continental drift, Wegener came
across a paleontological paper suggesting that a land bridge had once connected Africa with Brazil. This
proposed land bridge was an attempt to explain the well known paleontological observation that the same
fossilized plants and animals from the same time period were found in South America and Africa. The same
was true for fossils found in Europe and North America, and Madagascar and India. Many of these organisms
could not have traveled across the vast oceans that currently exist. Wegener's drift theory seemed more
plausible than land bridges connecting all of the continents. But that in itself was not enough to support his idea.
Another observation favoring continental drift was the presence of evidence for continental glaciation in the
Pensylvanian period. Striae left by the scraping of glaciers over the land surface indicated that Africa and South
America had been close together at the time of this ancient ice age. The same scraping patterns can be found
along the coasts of South America and South Africa.
Wegener's drift hypothesis also provided an alternate explanation for the formation of mountains (orogenesis).
The theory being discussed during his time was the "Contraction theory" which suggested that the planet was
once a molten ball and in the process of cooling the surface cracked and folded up on itself. The big problem
with this idea was that all mountain ranges should be approximately the same age, and this was known not to be
true. Wegener's explanation was that as the continents moved, the leading edge of the continent would
encounter resistance and thus compress and fold upwards forming mountains near the leading edges of the
drifting continents. The Sierra Nevada mountains on the Pacific coast of North America and the Andes on the
coast of South America were cited. Wegener also suggested that India drifted northward into the asian
continent thus forming the Himalayas.
Wegener eventually proposed a mechanism for continental drift that focused on his assertion that the rotation of
the earth created a centrifugal force towards the equator. He believed that Pangaea originated near the south
pole and that the centrifugal force of the planet caused the protocontinent to break apart and the resultant
continents to drift towards the equator. He called this the "pole-fleeing force". This idea was quickly rejected
by the scientific community primarily because the actual forces generated by the rotation of the earth were
calculated to be insufficient to move continents. Wegener also tried to explain the westward drift of the
Americas by invoking the gravitational forces of the sun and the moon, this idea was also quickly rejected.
Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the
prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories.
In 1929, about the time Wegener's ideas began to be dismissed, Arthur Holmes elaborated on one of Wegener's
many hypotheses; the idea that the mantle undergoes thermal convection. This idea is based on the fact that as a
substance is heated its density decreases and rises to the surface until it is cooled and sinks again. This repeated
heating and cooling results in a current which may be enough to cause continents to move. Arthur Holmes
suggested that this thermal convection was like a conveyor belt and that the upwelling pressure could break
apart a continent and then force the broken continent in opposite directions carried by the convection currents.
This idea received very little attention at the time.
Not until the 1960's did Holmes' idea receive any attention. Greater understanding of the ocean floor and the
discoveries of features like mid-oceanic ridges, geomagnetic anomalies parallel to the mid-oceanic ridges, and
the association of island arcs and oceanic trenches occurring together and near the continental margins,
suggested convection might indeed be at work. These discoveries and more led Harry Hess (1962) and R.Deitz
(1961) to publish similar hypotheses based on mantle convection currents, now known as "sea floor spreading".
This idea was basically the same as that proposed by Holmes over 30 years earlier, but now there was much
more evidence to further develop and support the idea.
Plate Tectonics: The Mechanism
The main features of plate tectonics are:
The Earth's surface is covered by a series of crustal plates.
The ocean floors are continually moving, spreading from the center, sinking at the edges, and being
regenerated.
Convection currents beneath the plates move the crustal plates in different directions.
The source of heat driving the convection currents is radioactivity deep in the Earths mantle.
Advances in sonic depth recording during World War II and the subsequent development of the nuclear
resonance type magnometer (proton-precession magnometer) led to detailed mapping of the ocean floor and
with it came many observation that led scientists like Howard Hess and R. Deitz to revive Holmes' convection
theory. Hess and Deitz modified the theory considerably and called the new theory "Sea-floor Spreading".
Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep
sea trenches, island arcs, geomagnetic patterns, and fault patterns.
Mid-Oceanic Ridges
The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide
surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater
mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000
meters deep. Research into the heat flow from the ocean floor during the early 1960s revealed that the greatest
heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic
ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity
at the mid-oceanic ridges.
Geomagnetic Anomalies
Occasionally, at random intervals, the Earth's magnetic field reverses. New rock formed from magma records
the orientation of Earth's magnetic field
at the time the magma cools. Study of the
sea floor with magnometers revealed
"stripes" of alternating magnetization
parallel to the mid-oceanic ridges. This is
evidence for continuous formation of
new rock at the ridges. As more rock
forms, older rock is pushed farther away
from the ridge, producing symmetrical
stripes to either side of the ridge. In the
diagram to the right, the dark stripes
represent ocean floor generated during
"reversed" polar orientation and the lighter stripes represent the polar orientation we have today. Notice that the
patterns on either side of the line representing the mid-oceanic ridge are mirror images of one another. The
shaded stripes also represent older and older rock as they move away from the mid-oceanic ridge. Geologists
have determined that rocks found in different parts of the planet with similar ages have the same magnetic
characteristics.
Deep Sea Trenches
The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface.
These trenches are usually long and narrow, and run parallel to and near the oceans margins. They are often
associated with and parallel to large continental mountain ranges. There is also an observed parallel association
of trenches and island arcs. Like the mid-oceanic ridges, the trenches are seismically active, but unlike the
ridges they have low levels of heat flow. Scientists also began to realize that the youngest regions of the ocean
floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the
ridges increased. In addition, it has been determined that the oldest seafloor often ends in the deep-sea trenches.
Island Arcs
Chains of islands are found throughout the oceans and especially in the western Pacific margins; the Aleutians,
Kuriles, Japan, Ryukus, Philippines, Marianas, Indonesia, Solomons, New Hebrides, and the Tongas, are some
examples.. These "Island arcs" are usually situated along deep sea trenches and are situated on the continental
side of the trench.
These observations, along with many other studies of our planet, support the theory that underneath the Earth's
crust (the lithosphere: a solid array of plates) is a malleable layer of heated rock known as the asthenosphere
which is heated by radioactive decay of elements such as Uranium, Thorium, and Potassium. Because the
radioactive source of heat is deep within
the mantle, the fluid asthenosphere
circulates as convection currents
underneath the solid lithosphere. This
heated layer is the source of lava we see
in volcanos, the source of heat that drives
hot springs and geysers, and the source of
raw material which pushes up the midoceanic ridges and forms new ocean
floor. Magma continuously wells
upwards at the mid-oceanic ridges (arrows) producing currents of magma flowing in opposite directions and
thus generating the forces that pull the sea floor apart at the mid-oceanic ridges. As the ocean floor is spread
apart cracks appear in the middle of the ridges allowing molten magma to surface through the cracks to form the
newest ocean floor. As the ocean floor moves away from the mid-oceanic ridge it will eventually come into
contact with a continental plate and will be subducted underneath the continent. Finally, the lithosphere will be
driven back into the asthenosphere where it returns to a heated state.
Continental drift was hotly debated off and on for decades following Wegener's death before it was largely
dismissed as being eccentric, preposterous, and improbable. However, beginning in the 1950s, a wealth of new
evidence emerged to revive the debate about Wegener's provocative ideas and their implications. In particular,
four major scientific developments spurred the formulation of the plate-tectonics theory: (1) demonstration of
the ruggedness and youth of the ocean floor; (2) confirmation of repeated reversals of the Earth magnetic field
in the geologic past; (3) emergence of the seafloor-spreading hypothesis and associated recycling of oceanic
crust; and (4) precise documentation that the world's earthquake and volcanic activity is concentrated along
oceanic trenches and submarine mountain ranges.
Ocean floor mapping
About two thirds of the Earth's surface lies beneath the oceans. Before the 19th century, the depths of the open
ocean were largely a matter of speculation, and most people thought that the ocean floor was relatively flat and
featureless. However, as early as the 16th century, a few intrepid navigators, by taking soundings with hand
lines, found that the open ocean can differ considerably in depth, showing that the ocean floor was not as flat as
generally believed. Oceanic exploration during the next centuries dramatically improved our knowledge of the
ocean floor. We now know that most of the geologic processes occurring on land are linked, directly or
indirectly, to the dynamics of the ocean floor.
"Modern" measurements of ocean depths greatly increased in the 19th century, when deep-sea line soundings
(bathymetric surveys) were routinely made in the Atlantic and Caribbean. In 1855, a bathymetric chart
published by U.S. Navy Lieutenant Matthew Maury revealed the first evidence of underwater mountains in the
central Atlantic (which he called "Middle Ground"). This was later confirmed by survey ships laying the transAtlantic telegraph cable. Our picture of the ocean floor greatly sharpened after World War I (1914-18), when
echo-sounding devices -- primitive sonar systems -- began to measure ocean depth by recording the time it took
for a sound signal (commonly an electrically generated "ping") from the ship to bounce off the ocean floor and
return. Time graphs of the returned signals revealed that the ocean floor was much more rugged than previously
thought. Such echo-sounding measurements clearly demonstrated the continuity and roughness of the
submarine mountain chain in the central Atlantic (later called the Mid-Atlantic Ridge) suggested by the earlier
bathymetric measurements.
In 1947, seismologists on the U.S. research ship Atlantis found that the sediment layer on the floor of the
Atlantic was much thinner than originally thought. Scientists had previously believed that the oceans have
existed for at least 4 billion years, so therefore the sediment layer should have been very thick. Why then was
there so little accumulation of sedimentary rock and debris on the ocean floor? The answer to this question,
which came after further exploration, would prove to be vital to advancing the concept of plate tectonics.
In the 1950s, oceanic exploration greatly expanded. Data gathered by oceanographic surveys conducted by
many nations led to the discovery that a great mountain range on the ocean floor virtually encircled the Earth.
Called the global mid-ocean ridge, this immense submarine mountain chain -- more than 50,000 kilometers
(km) long and, in places, more than 800 km across -- zig-zags between the continents, winding its way around
the globe like the seam on a baseball. Rising an average of about 4,500 meters(m) above the sea floor, the midocean ridge overshadows all the mountains in the United States except for Mount McKinley (Denali) in Alaska
(6,194 m). Though hidden beneath the ocean surface, the global mid-ocean ridge system is the most prominent
topographic feature on the surface of our planet.
Magnetic striping and polar reversals
Beginning in the 1950s, scientists, using magnetic instruments (magnetometers) adapted from airborne devices
developed during World War II to detect submarines, began recognizing odd magnetic variations across the
ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt -- the
iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral (magnetite) and can
locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th
century. More important, because the presence of magnetite gives the basalt measurable magnetic properties,
these newly discovered magnetic variations provided another means to study the deep ocean floor.
Early in the 20th century, paleomagnetists (those who study the Earth's ancient magnetic field) -- such as
Bernard Brunhes in France (in 1906) and Motonari Matuyama in Japan (in the 1920s) -- recognized that rocks
generally belong to two groups according to their magnetic properties. One group has so-called normal polarity,
characterized by the magnetic minerals in the rock having the same polarity as that of the Earth's present
magnetic field. This would result in the north end of the rock's "compass needle" pointing toward magnetic
north. The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the
Earth's present magnetic field. In this case, the north end of the rock's compass needle would point south. How
could this be? This answer lies in the magnetite in volcanic rock. Grains of magnetite -- behaving like little
magnets -- can align themselves with the orientation of the Earth's magnetic field. When magma (molten rock
containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is
"locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.
As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be
random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were
mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically
different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and
the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally
and reversely polarized rock, became known as magnetic striping.
Seafloor spreading and recycling of oceanic crust
The discovery of magnetic striping naturally prompted more questions: How does the magnetic striping pattern
form? And why are the stripes symmetrical around the crests of the mid-ocean ridges? These questions could
not be answered without also knowing the significance of these ridges. In 1961, scientists began to theorize that
mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along
the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually
erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading,
operating over many millions of years has built the 50,000 km-long system of mid-ocean ridges. This
hypothesis was supported by several lines of evidence: (1) at or near the crest of the ridge, the rocks are very
young, and they become progressively older away from the ridge crest; (2) the youngest rocks at the ridge crest
always have present-day (normal) polarity; and (3) stripes of rock parallel to the ridge crest alternated in
magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped
many times. By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge
system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the
development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a
natural "tape recording" of the history of the reversals in the Earth's magnetic field.
Additional evidence of seafloor spreading came from an unexpected source: petroleum exploration. In the years
following World War II, continental oil reserves were being depleted rapidly and the search for offshore oil was
on. To conduct offshore exploration, oil companies built ships equipped with a special drilling rig and the
capacity to carry many kilometers of drill pipe. This basic idea later was adapted in constructing a research
vessel, named the Glomar Challenger, designed specifically for marine geology studies, including the collection
of drill-core samples from the deep ocean floor. In 1968, the vessel embarked on a year-long scientific
expedition, criss-crossing the Mid-Atlantic Ridge between South America and Africa and drilling core samples
at specific locations. When the ages of the samples were determined by paleontologic and isotopic dating
studies, they provided the clinching evidence that proved the seafloor spreading hypothesis.
A profound consequence of seafloor spreading is that new crust was, and is now, being continually created
along the oceanic ridges. This idea found great favor with some scientists who claimed that the shifting of the
continents can be simply explained by a large increase in size of the Earth since its formation. However, this socalled "expanding Earth" hypothesis was unsatisfactory because its supporters could offer no convincing
geologic mechanism to produce such a huge, sudden expansion. Most geologists believe that the Earth has
changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question: how can new
crust be continuously added along the oceanic ridges without increasing the size of the Earth?
This question particularly intrigued Harry H. Hess, a Princeton University geologist and a Naval Reserve Rear
Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term
seafloor spreading. Dietz and Hess were among the small handful who really understood the broad implications
of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be
shrinking elsewhere. He suggested that new oceanic crust continuously spread away from the ridges in a
conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic
trenches -- very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic
Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the
trenches, new magma rose and erupted along the spreading ridges to form new crust. In effect, the ocean basins
were perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere
occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor
spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much
younger than continental rocks.
Concentration of earthquakes
During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording
instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in
certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists
were beginning to identify several prominent earthquake zones parallel to the trenches that typically were
inclined 40-60° from the horizontal and extended several hundred kilometers into the Earth. These zones later
became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first
recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity
greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network
(WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The
much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of
earthquake concentration worldwide.
But what was the significance of the connection between earthquakes and oceanic trenches and ridges? The
recognition of such a connection helped confirm the seafloor-spreading hypothesis by pin-pointing the zones
where Hess had predicted oceanic crust is being generated (along the ridges) and the zones where oceanic
lithosphere sinks back into the mantle (beneath the trenches).